1. Field of the Invention
This invention relates to a disc-shaped recording medium in which a recording track(s) are divided circumferentially to form a plurality of sectors in each of which track addresses are pre-recorded, and an apparatus for reproducing the track addresses on the disc-shaped recording medium.
2. Description of the Prior Art
Heretofore, an optical disc having a disc substrate at least one major surface of which is provided with a recording layer which is responsive to light to record information signals thereon, and an optical recording and/or reproducing apparatus having the optical disc as a recording medium, have been put to practical use.
As this type of the optical disc, the playback-only ROM disc (CD) and the magneto-optical disc on which data can be re-written by the user, are well-known.
The above-mentioned magneto-optical disc is usually provided with concentric recording tracks or a spiral recording track in its recording region, which track(s) are divided circumferentially for dividing each track turn into plural sectors.
Data recorded in each sector are divided into e.g. 32 segments, as shown in FIG. 1a. The leading end of each segment is formed as a servo byte area (SB) in which sample servo pits are pre-recorded, such as by embossing.
An address information is recorded in a header/1 at the leading end of each sector, and an automatic laser power control area (ALPC area) is provided in the trailing side of each sector. A test-write area for tentative data writing is provided in the next header/2.
The header/2 is followed by 30 segments S.sub.G1 to S.sub.G30, capable of recording input data.
As shown to an enlarged scale in FIG. 1b, the header/1 is provided first with the servo byte area SB, followed by a track address TA consisting of a 10-byte word, indicating a track address, and by a 2-byte sector address SA, indicating a sector mark.
In the servo byte area SB of each segment in each track, there is provided a hexadecimal Gray code GC, indicating the relative position of the track, for being used as an access code when the code is traversed by an optical head. In the servo byte area, there are also provided wobbling pits P.sub.1 and P.sub.3, providing the tracking information for the optical head, and a clock pit P.sub.2 for providing the clocking information.
As data indicating the track address in the header/1, 5-word data, i.e. 5 bytes of data, including the most significant bytes (MSB), the second most significant bytes (2SB), the third most significant bytes (3SB), the fourth most significant bytes (4SB), and the least significant bytes (LSB) of data are recorded as a track address 1. Meanwhile, parity check words which are complements relative to these five words are recorded as a track address 2 in continuation to these words.
When a desired track on the optical disc, on which the above-mentioned address information is recorded, is accessed, starting from the current track, the current traversing position may be grasped by reading the track address at the leading end of each sector and then the Gray code at the leading end of each segment indicating the lower order bits of the track address.
As the optical head becomes increasingly lightweight and the seek speed is increased, it has become possible for a laser beam to traverse the optical head radially at an increased velocity, so that it has become difficult to correctly read out the track address which is recorded in the header/1 and met in the course of a seeking operation. If the track address is misread, accessing cannot be achieved smoothly, so that it becomes impossible to achieve the high speed seeking.
On the other hand, if the absolute address of the track at the end of the seeking cannot be read, smooth landing on the target track is inhibited to lengthen the seeking time.
It has been attempted to record part of the address information recorded on the header/1 by the Gray code so that the track position will not be different significantly on occurrence of data readout errors. However, there is still the risk that the track position may be mistaken significantly when the optical head traverses the track where a bit shift in the higher position direction or in the lower position direction occurs between the track address words.